If statements vs conditions

Question

Joy Shen 2023-9-6

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/2017671-if-statements-vs-conditions

评论： Joy Shen 2023-9-7

采纳的回答： Voss

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Hi all,

I'm trying to set conditions to define some later operations.

I'm essentially simulating Esim4 which can be a value from 0 to 10, and fgsim which is a binary variable indicating whether something is installed (1) or missing (0). I'm then trying to calculate Asim_door. If fg installed (i.e., fgsim=1) and Esim4 is below n_fg, then Asim_door=0. If fg is installed and Esim4 is above n_fg, then Asim_door=Asim_door/A_fg. Otherwise if fg is not installed then Asim_door is not changed.

I get an error "Unable to perform assignment because the left and right sides have a different number of elements." when I attempt this with conditions as indicated by the %%%%.

What's the difference between doing it this way and an if statement?

% CVol3 sim
for iCombCVol3=1:size(IndMatCVol3) %Parent PS1 has 3 states for child node Vi
    % EFH loc 4 sim
    EFH4_a=[]; EFH4_a=ExtSurgeBinEdges_Lo(IndMatCVol3(iCombCVol3,4));
    EFH4_c=[]; EFH4_c=ExtSurgeBinEdges_Hi(IndMatCVol3(iCombCVol3,4));
    pdEFH4=[]; pdEFH4=makedist('Uniform',EFH4_a,EFH4_c);
    Esim4=[]; Esim4=random(pdEFH4,[Nsim,1]); %simulate external flood heights within bin (Uniform)
    Esim4=Esim4.*(Esim4>=0); % make sure the difference is never below zero
    
    % Cumulative flow volume 1 sim
    CVol2_a=[]; CVol2_a=VBinEdges_Lo(IndMatCVol3(iCombCVol3,3)); 
    CVol2_c=[]; CVol2_c=VBinEdges_Hi(IndMatCVol3(iCombCVol3,3)); 
    pdCVol2=[]; pdCVol2=makedist('Uniform',CVol2_a,CVol2_c);
    CVol2sim=[]; CVol2sim=random(pdCVol2,[Nsim,1]); %simulate external flood heights within bin (Uniform)
    CVol2sim=CVol2sim.*(CVol2sim>=0); % make sure the difference is never below zero
    CVol2sim(CVol2sim>=Vroom(:,1))=Vroom(:,1); % make sure the difference is never below zero
    
    %fgsim installed or missing
    p_fg = [0.97 0.03]; % prob of fg installed, missing
    % fg state sim of installed or not
    pd_fg = makedist('Binomial',1, p_fg(IndMatCVol3(iCombCVol3,2)));
    fgsim = random(pd_fg,[Nsim,1]);
    % Door state sim through area
    Asim_door = Esim4*door_W;
    Asim_door(Asim_door<0)=0; % make sure the area is postive
    Asim_door(Asim_door>A_door)=A_door;  %make sure area is never bigger than the possible area of door
    %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
    Asim_door(fgsim>=1 & Esim4<=n_fg)=0; %fg installed, Esim below n_fg then no A_door flow area available
    Asim_door(fgsim>=1 & Esim4>=n_fg)=(Asim_door./A_fg); %fg installed, Esim above n_fg then A_door/A_fg flow area available
    %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
    
    % Volumetric flow rate 2 sim calculation
    Qsim_door=[]; Qsim_door=Cd.*Asim_door.*sqrt(2*g*(Esim4)); %ft^3/s
    Qsim_door=Qsim_door.*(Qsim_door>=0); %Ensures positivity
    Qsim_door(Esim4<=n_ps)=1E-20; %No flow when Esim is less than or equal to installation height
    
    % Flow volume sim calculation
    Vsim_door=[]; Vsim_door=Qsim_door.*Dsim; %ft^3
    Vsim_door=Vsim_door.*(Vsim_door>=0); %ensure positivity
    Vsim_door(Vsim_door>=Vroom(:,1))=Vroom(:,1); %Ensure Vsim is less than Vroom (3 Vroom configs)
    Vsim_door(Esim4<=n_ps)=1E-20; %No flow volume when Esim is less than or equal to installation height
    
    % Cumulative flow 1 sim calculation
    CVol3sim= CVol2sim+Vsim_door;
    CVol3sim=CVol3sim.*(CVol3sim>=0); %ensure positivity
    CVol3sim(CVol3sim>=Vroom(:,1))=Vroom(:,1);
    CVol3sim(CVol2sim<=0 & Vsim_door<=0)=0; % No cumulative flow 3 if CVol2 and Vsim_door have no flow
    
    % Histogram
    Hist_CVol3=[]; [Hist_CVol3,~] = histcounts(CVol3sim,VBinEdges);
    Hist_CVol3=Hist_CVol3';
    PMF_CVol3(:,iCombCVol3)=Hist_CVol3./sum(Hist_CVol3); % Bins it by the parents    
end

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Answer 1

Voss 2023-9-6

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/2017671-if-statements-vs-conditions#answer_1303486

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In this line, you have the entire array Asim_door (divided by A_fg) on the right side, trying to assign it to a subset on the left side. That's the problem.

Asim_door(fgsim>=1 & Esim4>=n_fg)=(Asim_door./A_fg); %fg installed, Esim above n_fg then A_door/A_fg flow area available

You have to use the same indices on both sides.

So if A_fg is a scalar:

idx = fgsim>=1 & Esim4>=n_fg;
Asim_door(idx) = Asim_door(idx)./A_fg; %fg installed, Esim above n_fg then A_door/A_fg flow area available

Or if A_fg is an array the same size as Asim_door:

idx = fgsim>=1 & Esim4>=n_fg;
Asim_door(idx) = Asim_door(idx)./A_fg(idx); %fg installed, Esim above n_fg then A_door/A_fg flow area available

What's the difference between doing it this way and an if statement?

Doing it this way (using logical indexing) is more concise and likely to be faster than using an if statement. With an if, you'd need to loop over the elements, e.g., if A_fg, n_fg and fgsim are arrays the same size as Asim_door:

for ii = 1:numel(Esim4)
    if fgsim(ii) >= 1 && Esim4(ii) >= n_fg(ii)
        Asim_door(ii) = Asim_door(ii)./A_fg(ii);
    end
end

and if any of them are scalars, remove the (ii) indexing for those (and you could likely simplify the logic, e.g., if fg_sim is a scalar, you do not need to perform the loop at all if fg_sim < 1).

Note that the logical indexing expression

idx = fgsim>=1 & Esim4>=n_fg;

works the same regardless of whether n_fg and/or fgsim are scalars because of how & works:

true & [false false true]
ans = 1×3 logical array
   0   0   1
[true true true] & [false false true]
ans = 1×3 logical array
   0   0   1

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Joy Shen 2023-9-7

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Thanks @Voss, this helped with my code. Are there rules of thumb for the order of these conditional statements? Here's my updated code. For each Volume and Area I've restricted them from 0 to the maximum area. I wasn't the tidiest with order, but I wasn't sure if it mattered that much. This is still giving me strange results so I figured I'd ask.

for iCombCVol3=1:size(IndMatCVol3) %Parent PS1 has 3 states for child node Vi
    % EFH loc 4 sim
    EFH4_a=[]; EFH4_a=ExtSurgeBinEdges_door_Lo(IndMatCVol3(iCombCVol3,4));
    EFH4_c=[]; EFH4_c=ExtSurgeBinEdges_door_Hi(IndMatCVol3(iCombCVol3,4));
    pdEFH4=[]; pdEFH4=makedist('Uniform',EFH4_a,EFH4_c);
    Esim4=[]; Esim4=random(pdEFH4,[Nsim,1]); %simulate external flood heights within bin (Uniform)
    Esim4=Esim4.*(Esim4>=0); % Ensure positivity
    
    % Cumulative flow volume 1 sim
    CVol2_a=[]; CVol2_a=VBinEdges_Lo(IndMatCVol3(iCombCVol3,3)); 
    CVol2_c=[]; CVol2_c=VBinEdges_Hi(IndMatCVol3(iCombCVol3,3)); 
    pdCVol2=[]; pdCVol2=makedist('Uniform',CVol2_a,CVol2_c);
    CVol2sim=[]; CVol2sim=random(pdCVol2,[Nsim,1]); %simulate external flood heights within bin (Uniform)
    CVol2sim=CVol2sim.*(CVol2sim>=0); % Ensure positivity
    CVol2sim(CVol2sim>=Vroom(:,1))=Vroom(:,1); % Cap it at room height
    %fgsim installed or missing
    p_fg = [0.97 0.03]; % prob of fg installed, missing
    % fg state sim of installed or not
    pd_fg = makedist('Binomial',1, 0.97);%Probability of fgsim installed %p_fg(IndMatCVol3(iCombCVol3,2)));
    fgsim = random(pd_fg,[Nsim,1]);
    % Door state sim through flow area with fg installed
    Mu_door_fg=[]; Mu_door_fg=Amean_door(IndMatCVol3(iCombCVol3,1))-Amean_fg(IndMatCVol3(iCombCVol3,1));
    Sig_door_fg=[]; Sig_door_fg=0.2*Mu_door_fg;
    pd_Asim_door_fg=[]; pd_Asim_door_fg = makedist('normal',Mu_door_fg,Sig_door_fg);
    Asim_door_fg = []; Asim_door_fg=random(pd_Asim_door_fg,[Nsim,1]);
    Asim_door_fg(Esim4<=n_fg)=0;
    
    % Door state sim through flow area with fg missing
    Mu_door=[]; Mu_door=Amean_door(IndMatCVol3(iCombCVol3,1));
    Sig_door=[]; Sig_door=0.2*Mu_door;
    pd_Asim_door=[]; pd_Asim_door = makedist('normal',Mu_door,Sig_door);
    Asim_door = []; Asim_door=random(pd_Asim_door,[Nsim,1]);
    
    Asim_door(fgsim==1)=Asim_door_fg(fgsim==1); %When fgsim is installed, replace values with Asim_door_fg
    Asim_door(fgsim==1 & Esim4<=n_fg)=0; %fg installed, Esim below n_fg then no A_door flow area available
    Asim_door=Asim_door.*(Asim_door>=0); %Ensures positivity
    Asim_door(Asim_door>=A_door)=A_door; % Cap it at door area
    % Volumetric flow rate 2 sim calculation
    Qsim_door=Cd.*Asim_door.*sqrt(2*g*Esim4);
    Qsim_door_fg=Cd.*Asim_door_fg.*sqrt(2*g*(Esim4-n_fg)); %ft^3/s
    Qsim_door_fg(Esim4<n_fg)=0;
    Qsim_door(fgsim==1)=Qsim_door_fg(fgsim==1); %When fgsim is installed, replace values with Qsim_door_fg
    Qsim_door(fgsim==1 & Esim4<=n_fg)=0; % Fg installed, no flow when Esim is less than or equal to installation height
    Qsim_door=Qsim_door.*(Qsim_door>=0); %Ensures positivity
    
    % Flow volume sim calculation
    Vsim_door=[]; Vsim_door=Qsim_door.*Dsim; %ft^3
    Vsim_door=Vsim_door.*(Vsim_door>=0); %ensure positivity
    Vsim_door(fgsim==1 & Esim4<=n_fg)=0; %No flow volume when Esim is less than or equal to installation height
    Vsim_door(Vsim_door>=Vroom(:,1))=Vroom(:,1); %Ensure Vsim is less than Vroom (3 Vroom configs)
    
    % Cumulative flow 1 sim calculation
    CVol3sim= CVol2sim+Vsim_door;
    CVol3sim=CVol3sim.*(CVol3sim>=0); %ensure positivity
    CVol3sim(CVol2sim<=0 & Vsim_door<=0)=0; % No cumulative flow 3 if CVol2 and Vsim_door have no flow
    CVol3sim(CVol3sim>=Vroom(:,1))=Vroom(:,1); % Caps at the room volume
    % Histogram
    Hist_CVol3=[]; [Hist_CVol3,~] = histcounts(CVol3sim,VBinEdges);
    Hist_CVol3=Hist_CVol3';
    PMF_CVol3(:,iCombCVol3)=Hist_CVol3./sum(Hist_CVol3); % Bins it by the parents    
end

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Answer 2

Steven Lord 2023-9-6

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链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/2017671-if-statements-vs-conditions#answer_1303506

在 MATLAB Online 中打开

Esim4=Esim4.*(Esim4>=0); % make sure the difference is never below zero

I likely would just call the max function on Esim4 instead.

Esim4 = max(Esim4, 0);

Some of your other conditional steps could probably be replaced by calls to min and/or max instead of logical indexing (potentially combined with element-wise multiplication.)